Previously, insulin receptors could be demonstrated in membrane preparations of heads and bodies of chick embryo by 2 days of incubation. Binding of labeled insulin to brain and liver membranes from day 8 to 18 embryos increases with time of development. Recently, insulin-like growth factor binding was studied in chick embryo tissue. Using labeled insulin-like growth factor I and II (multiplication stimulating activity) and unlabeled homologous and heterologous peptides in competion binding assays, it appears that there are a specific insulin-like growth factor I but not insulin-like growth factor II receptors in the developing chick embryo. The growth factor binding pattern is different from insulin binding in several chick embryo tissues. Previously, we have demonstrated that insulin is present in chick embryos at day 2-3 as well as unfertilized eggs. To define insulin's role in early development anti insulin antibodies were injected into fertilized eggs and the effect of antibodies was studied. Antibody treated embryos had a higher rate of growth retardation and death by days 3-5 of embryogenesis; surviving embryos had delayed biochemical maturation compared to controls. Insulin receptor and insulin-like growth factors (I and II) receptors structure studies have been extended in rat brain. The binding of labeled peptides to thin sections of frozen fresh rat brain was visualized with autoradiography. By several criteria including structure-activity relationship analysis, these brain peptide receptors were qualitatively indistinguishable from peptide receptors previously characterized on brain and other more typical target tissues and distinct from each other. Each peptide exhibits its own distinctive binding pattern - i.e. each peptide binds to cytoarchetectonic structures. In addition, all three peptides demonstrate a high density of binding to choriod plexus.